This project will investigate the factors regulating synthesis, secretion and metabolism of triiodothyronine (T3) and thyroxine (T4) and evaluate the relative biological significance of these two hormones. Qualitative and quantitative aspects of thyroid hormonogenesis and secretion will be investigated using TSH stimulation. Correlation of 127I content will be made before and after TSH or TRH in animals using immunoassays of T4 and T4 in pronase digests of thyroglobulin. Animal thyroids will be prelabelled at various intervals with 131I and 125I. Specific T3 antibody-sepharose conjugates will be used for immunoadsorption of labelled T3 from the digest extracts. The varying affinities of T3 and T4 for the circulating serum thyroxine-binding proteins result in different metabolic clearance rates for the two hormones and differences in partition between intracellular and extracellular sites. To determine the relative contribution of T3 and T4, they will be quantitated using both immunassay and tracer techniques. The mechanism(s) for the control of peripheral production of T3 and T4 are not known, yet there is suggestive evidence that this is a specific process. The biological implications of this conversion will be investigated using hypothyroid animals given T3 and T4. Propylthiouracil will be used to inhibit T4 to T3 conversion. Because of the short half-life of T3, constant infusions and TRH will be used in rats for precise determination of the physiological role played by this hormone in suppression of TSH synthesis and/or release.